The mm-wave region of the electromagnetic spectrum (defined herein to mean 30 GHz up to 1 THz) is still unexploited in high-power RF devices, mainly because of the lack of phased-locked sources that are able to provide substantial amount of power. Traditional linear interaction RF sources, such as Klystrons and Traveling Wave Tubes, fail to produce significant power levels at this part of the frequency spectrum. This is because their critical dimensions are small compared to the wavelength, and therefore the amount of beam current that can go through the beam apertures is very limited. There is therefore a need for compact, high power mm-wave sources. These would also enable several additional applications such as basic research, high-resolution medical imaging, navigation through sandstorms, spectroscopic detection of explosives, high bandwidth, low probability of intercept communications, space radars for debris tracking of objects less that 5 cm that present hazards to space assets such as communications satellites, and even human space flight safety in the future.